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United States Patent |
6,095,251
|
Mitchell
,   et al.
|
August 1, 2000
|
Dual stage fire extinguisher
Abstract
A fire is extinguished and suppressed, by a dual stage fire extinguisher.
In a first stage, a sufficient amount of an inerting agent is delivered to
extinguish the fire. Once the fire is extinguished, in the second stage, a
different amount of inerting agent is delivered to the fire to prevent its
re-ignition. Since suppression typically requires less of the inerting
agent than extinguishing, a reduction in the weight of the inerting agent
is achieved with the dual stage process making the system particularly
amenable to aircraft applications such as in an engine nacelle or cargo
dry bay.
Inventors:
|
Mitchell; Robert M. (Issaquah, WA);
Wierenga; Paul H. (Seattle, WA);
Hoskins; Randel L. (Bothell, WA)
|
Assignee:
|
Primex Technologies, Inc. (Redmond, WA)
|
Appl. No.:
|
115190 |
Filed:
|
July 14, 1998 |
Current U.S. Class: |
169/43; 169/26; 169/44; 169/45; 169/46; 169/54; 169/62 |
Intern'l Class: |
A62C 002/00 |
Field of Search: |
169/5,26,43,44,45,46,54,62
|
References Cited
U.S. Patent Documents
2015995 | Oct., 1935 | Egtvedt | 169/62.
|
3356148 | Dec., 1967 | Jamison | 169/44.
|
3388746 | Jun., 1968 | Linderberg | 169/62.
|
3584688 | Jun., 1971 | Duncan et al. | 169/26.
|
5188257 | Feb., 1993 | Plester | 169/44.
|
5393438 | Feb., 1995 | Fernandez | 169/46.
|
5449041 | Sep., 1995 | Galbraith | 169/26.
|
5909776 | Jun., 1999 | Stewart et al. | 169/26.
|
Primary Examiner: Kashnikow; Andres
Assistant Examiner: Evans; Robin O.
Attorney, Agent or Firm: Wiggin & Dana, Rosenblatt; Gregory S.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a continuation-in-part of U.S. patent application Ser. No.
09/034,711, filed Mar. 4, 1998, which claims priority to U.S. Provisional
Patent Application Ser. No. 60/053,365, filed Jul. 22, 1997.
Claims
We claim:
1. A method for inerting a fire at a location, comprising the steps of:
a) providing a first chamber containing a first body of a first inerting
agent and a second chamber containing a second body of a second inerting
agent, the first and second bodies initially discrete;
b) delivering the first inerting agent along a flow path from the first
chamber to said location at a first mass flow rate for a time effective to
extinguish said fire; and
c) delivering the second inerting agent from the second chamber through the
first chamber and along said flow path to said location at a second mass
flow rate for a time effective to prevent re-ignition of said fire.
2. The method as claimed in claim 1 further comprising:
providing a first pressurant in the first chamber and a second pressurant
in the second chamber.
3. The method as claimed in claim 2 wherein the first inerting agent is
delivered as a liquid and the second inerting agent is delivered as a gas.
4. The method as claimed in claim 3 wherein the first inerting agent is
water-based and the second inerting agent is an HFC.
5. The method as claimed in claim 4 wherein the second inerting agent is
selected from the group consisting of HFC-125, HFC-23, HFC-227ea, and
HFC-236fa.
6. With an aircraft fire extinguishing/suppressing system for delivering
inerting agent along a flow path from a storage location in the aircraft
to a fire location in the aircraft, a method for rebuilding the system
comprising the steps of:
removing a first vessel containing a single body of inerting agent from the
storage location;
thereafter installing in the storage location a second vessel containing
first and second discrete bodies of inerting agent respectively in first
and second chambers within the second vessel and coupling the second
vessel to a conduit so that the conduit, first chamber and second chamber
are located in series so that upon actuation of the system with the second
vessel installed inerting agent is sequentially delivered: from the first
body in the first chamber along the flow path to the fire location; and
then from the second body in the second chamber, through the first
chamber, and along the flow path to the fire location.
7. The method as claimed in claim 6 wherein the effective amount of said
single inerting agent in said single body is larger than a combined
effective amount of inerting agent in said first and second bodies.
8. The method as claimed in claim 7 wherein throttling is provided between
the first and second chambers effective to restrict flow of inerting agent
from the second chamber to the first chamber so that upon actuation of the
system with the second vessel installed, discharge of inerting agent
occurs over a second interval of time which is longer than a first
interval of time required for discharge of inerting agent with the first
vessel installed.
9. The method as claimed in claim 6 wherein the first inerting agent is
delivered as a liquid and the second inerting agent is delivered as a gas.
10. The method as claimed in claim 9 wherein the first inerting agent is
water-based and the second inerting agent is an HFC.
11. The method as claimed in claim 9 wherein the second inerting agent is
selected from the group consisting of HFC-125, HFC-23, HFC-227ea, and
HFC-236fa.
12. An apparatus for extinguishing a fire and suppressing re-ignition
thereof, comprising:
a first chamber for containing a first inerting agent;
a second chamber for containing a second inerting agent; and
a common conduit to substantially sequentially direct the first inerting
agent and the second inerting agent to a fire location such that the first
inerting agent is introduced to the fire location during an interval
effective to extinguish the fire and the second inerting agent is
introduced to the fire location during an interval effective to suppress
re-ignition of the fire,
wherein the first chamber is defined by a downstream portion of a vessel
and the second chamber is defined by an upstream portion of the vessel,
the upstream portion separated from the downstream portion by a dividing
wall spanning the vessel, the wall including an openable aperture for
permitting flow of the second inerting agent from the second chamber into
the first chamber responsive to an at least partial depletion of the first
inerting agent from the first chamber.
13. The apparatus as claimed in claim 12 wherein said first inerting agent
is the same chemical compound as said second inerting agent.
14. The apparatus of claim 12 further including a first pressurant within
the first chamber and a second pressurant within the second chamber.
15. The apparatus of claim 14 wherein the aperture is initially sealed by a
burst valve, which automatically opens when pressure in the second chamber
exceeds pressure in the first chamber by a threshold amount.
16. The apparatus as claimed in claim 12 wherein said first inerting agent
is a different chemical compound than said second inerting agent.
17. The apparatus as claimed in claim 12 wherein said first and second
inerting agents are selected from the group consisting of HFC-125, HFC-23,
HFC-227ea, and HFC-236fa.
18. An apparatus for extinguishing a fire and suppressing re-ignition
thereof, comprising:
a first chamber for containing a first inerting agent;
a second chamber for containing a second inerting agent; and
a common conduit extending downstream from the first chamber and coupled to
the second chamber by the first chamber to substantially sequentially
direct the first inerting agent and the second inerting agent to a fire
location such that the first inerting agent is introduced to the fire
location during an interval effective to extinguish the fire and the
second inerting agent is introduced to the fire location during an
interval effective to suppress re-ignition of the fire.
19. An apparatus for extinguishing a fire and suppressing re-ignition
thereof, comprising:
a first chamber defined by a first vessel for containing a first inerting
agent;
a second chamber defined by a second vessel for containing a second
inerting agent;
a baffle coupled to said first vessel and said second vessel for
controlling a flow rate of said first and second inerting agents; and
a common conduit coupled to the baffle to substantially sequentially direct
the first inerting agent and the second inerting agent to a fire location
such that the first inerting agent is introduced to the fire location
during an interval effective to extinguish the fire and the second
inerting agent is introduced to the fire location during an interval
effective to suppress reignition of the fire.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for extinguishing a fire and preventing
re-ignition. More particular, a fire extinguishing agent is discharged at
a first mass flow rate to extinguish the fire followed by discharge at a
second mass flow rate that is effective to prevent re-ignition of the
fire.
2. Description of Related Art
Fire involves a chemical reaction between oxygen and a fuel that is raised
to its ignition temperature by heat. Fire suppression systems operate by
any one or a combination of the following: (i) removing oxygen, (ii)
reducing the system temperature, (iii) separating the fuel from oxygen,
and (iv) interrupting the chemical reactions of combustion. Typical fire
suppression agents include water, carbon dioxide, dry chemicals,
perfluorocarbons (PFC's), hydrofluorocarbons (HFC's) and the group of
halo-carbons collectively known as Halons.
The most efficient fire suppression agents are Halons. Halons are a class
of brominated fluorocarbons and are derived from saturated hydrocarbons,
such as methane or ethane, with their hydrogen atoms replaced with atoms
of the halogen elements bromine, chlorine and/or fluorine. The most widely
used Halon is Halon 1301, CF.sub.3 Br, trifluorobromomethane. Halon 1301
extinguishes a fire in concentrations far below the concentrations
required for carbon dioxide or nitrogen gas. Typically, a Halon 1301
concentration above about 3.3% by volume will extinguish a fire.
Halon fire suppression occurs through a combination of effects, including
decreasing the available oxygen, isolation of fuel from atmospheric
oxygen, cooling and chemical interruption of the combustion reactions. The
superior fire suppression efficiency of Halon 1301 is due to its ability
to terminate the runaway reaction associated with combustion. The
termination step is catalytic for Halon 1301 due to the stability of
bromine radicals (Br.circle-solid.) formed when Halon 1301 is disposed on
a combustion source.
When unreacted Halon 1301 migrates into the stratosphere, sunlight breaks
down the Halon 1301 forming bromine radicals which react to consume ozone
in an irreversible manner:
Br.circle-solid.+O.sub.3 .fwdarw.Br.circle-solid.+O.sub.2
In view of the current recognition that ozone depletion is a serious
environmental problem, a move is on to: (i) identify fire suppression
agents having a less severe environmental impact than Halon; and (ii)
develop devices to deliver these more environmentally friendly agents.
Most agents identified as replacements for Halon 1301 are not as efficient
extinguishants. Typically, these replacement agents require between two
and three times the volume as compared to Halon 1301. The excess volume
creates a retrofit problem when space is at a premium.
In addition to extinguishing the fire, it is necessary to suppress the fire
as well. Suppression insures that the fire does not re-ignite and requires
an inerting agent to remain in contact with the location of the
extinguished fire for a time sufficient to either (1) reduce the system
temperature below the temperature necessary to support combustion, (2)
remove the fuel source, or (3) separate the fuel from the oxygen.
A fire suppression apparatus is frequently located in an aircraft engine
nacelle, the aerodynamic structure surrounding the engine. An annular
region between the engine and the nacelle presents a fire hazard. During
flight, all the requirements of a fire--fuel, oxygen and heat--are present
in the nacelle. Some aircraft engine components operate at elevated
temperatures, in excess of 700.degree. F. (370.degree. C.), and are thus
capable of igniting fuel. An airflow containing oxygen is routed through
the annular region to cool the engine. Fuel and hydraulic fluids are
supplied to the engine in lines that extend through the region and can
leak. In combat, military aircraft can be exposed to unfriendly fire that
can sever fuel or hydraulic lines as can other mechanical failures or
damage.
Therefore, most commercial and military aircraft utilize an on-board engine
nacelle fire detection and extinguishing/suppression system.
Conventionally, when a fire occurs in an engine nacelle, the pilot performs
two tasks to save the aircraft: (1) fuel to the engine is shut off; and
(2) an on-board fire extinguisher is activated discharging an agent into
the nacelle. In some aircraft, the fuel is automatically shut off to the
engine in question when the extinguisher is discharged. Generally, several
seconds are required to de-pressurize or bleed the fuel lines, during
which interval, they may continue to deliver fuel to the fire.
After the nacelle fire is extinguished, re-ignition must be prevented.
Preventing an extinguished fire from re-igniting is called suppression. If
the re-ignition source is a component operating at an elevated
temperature, the suppression time is dependent on how long it takes to
bleed the fuel out of the lines. If the re-ignition source is a surface
heated by the fire, then the suppression time is dependent on the time it
takes the air flow to cool the surface below the ignition temperature (if
less than the time required to bleed the fuel line). In either instance,
generally from about six to seven seconds are required to inert a fire,
extinguish it, and suppress its re-ignition. Therefore, the inerting agent
must be able to extinguish the fire and keep it out for a predetermined
time, which is typically aircraft-specific.
When used as an inerting agent, Halon 1301 is discharged from a pressurized
bottle. The bottle containing the Halon 1301 is supercharged with nitrogen
to a predetermined pressure. When activated, the agent is discharged by a
blowdown mode and routed to the nacelle via tubing. It is necessary to
maintain a minimum concentration of 3.3%, by volume, of Halon 1301 over
the entire time required to extinguish and suppress re-ignition of the
fire. To compensate for the dissipation of inerting agent, in a
conventional fire extinguisher the concentration of inerting agent is
initially brought up to a level significantly higher than 3.3% to insure
that an effective concentration will remain for suppression. The inventors
have observed that this excess amount of inerting agent is not required to
fight the fire and represents a significant penalty as to cost, weight and
environmental impact.
There remains, therefore, a need for a system to economically inert a fire
that does not suffer from the disadvantages of the prior art.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a method to
efficiently inert a fire. It is a feature of the invention that an
inerting media is delivered in two stages. In a first stage, a mass flow
rate effective to extinguish the fire is employed. In a second stage, the
inerting medium is delivered at a different mass flow rate that is
effective to suppress re-ignition of the fire.
Among the advantages of the method of the invention are that a minimum
quantity of inerting media for a given fire situation is employed. This
reduces the cost and the weight of the fire suppression system and, in the
instance of Halons and other environmentally destructive media, reduces
the environmental impact. Another advantage of the invention is that the
dual stage process is amenable to many types of fire suppression systems
and requires minimal retrofitting of existing equipment.
In accordance with the invention, there is provided a method to inert a
fire. The method includes delivering a first inerting agent to the fire. A
second inerting agent is then delivered to location of the extinguished
fire at a second mass flow rate for a time effective to prevent
re-ignition.
A system according to the invention may advantageously be configured to be
used in an application formerly served by a prior art system. This may
include a retrofit use such as to replace an existing system in an
existing aircraft. Relative to the replaced system, the replacement system
may have any or all of the following attributes:
a) The amount (either as an absolute percentage or a relative percentage)
by which the peak concentration of inerting agent exceeds a required
concentration will be lower in the replacement system. This may apply if
the inerting agent in the replacement system is the same as or different
from the inerting agent in the replaced system.
b) If the inerting agents are the same in the replacement and replaced
system (and thus the required concentration of agent in the system is the
same), the specific peak concentration in the replacement system will be
lower.
c) The amount of agent actually expended during the interval necessary to
extinguish and suppress re-ignition of the fire will be relatively closer
to the minimum required amount in the replacement system as compared with
the replaced system. The total effective amount of inerting agent (mass
multiplied by the efficiency of the particular agent) in the replacement
system will be less than that in the replaced system.
d) A relatively less efficient but more environmentally-safe inerting agent
may be used in the replacement system.
e) The expulsion of inerting agent will occur over a longer interval of
time in the replacement system preferably approximately co-extensive with
the predicted (including margin of error) interval necessary to extinguish
and suppress re-ignition of the fire.
The above stated objects, features and advantages will become more apparent
from the specification and drawings that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in partial breakaway view, an aircraft engine containing
a fire suppression system.
FIG. 2 graphically illustrates the mass flow rate of Halon 1301 as a
function of time when utilized according to prior art methods.
FIG. 3 graphically illustrates the concentration, in volume percent, of
Halon 1301 when utilized according to the method of the prior art.
FIG. 4 illustrates the mass flow rate of an inerting medium in accordance
with the invention.
FIG. 5 illustrates the concentration, in volume percent, of the inerting
medium in accordance with the method of the invention.
FIG. 6 illustrates the mass flow rate improvement achieved by the method of
the invention.
FIG. 7 illustrates the concentration in volume percentage improvement
achieved by the method of the invention.
FIG. 8 illustrates the weight savings achieved by the method of the
invention.
FIGS. 9-13 illustrate systems to deliver an inerting agent in accordance
with the method of the invention.
DETAILED DESCRIPTION
FIG. 1 illustrates an aircraft engine 10 including a core engine 12
supported by a fan nacelle 14 as illustrated in U.S. Pat. No. 5,239,817 to
Mildenstein et al. The aircraft engine 10 is a fan jet type and includes
rotating fan blades 16. A fan discharge 18 is in annular passageway
extending between an inner surface of the nacelle 14 and an outer surface
of the core engine nacelle 20. A core compartment 22 is defined as the
space between the inner surface of the core engine nacelle 20 and the
outer surface of the core engine 12. An inlet 24 introduces cooling air
through the engine compartment that exits through an outlet 26.
The engine 10 operates at elevated temperature and has a ready supply of
oxygen, through the cooling air. Therefore, if jet fuel or flammable
hydraulic liquids are discharged between the nacelle and the engine, a
fire is a definite possibility. To extinguish the fire, an inerting agent
28 housed remotely from the engine 10 is delivered to the engine through
conduit 30. Usually, conduit 30 ends at a plurality of discharge ports 32
disposed axially and circumferentially around the core engine 12.
FIGS. 2 and 3 graphically illustrate discharge characteristics typical for
a pressurized liquid inerting agent, such as Halon 1301. Reference line 34
is the agent mass flow rate and illustrates the delivery rate of the
inerting agent in pounds-mass per second. Reference line 36 illustrates
the total amount of inerting agent delivered to the fire in pounds. Region
38 identifies when the fire is extinguished and region 40 identifies when
the fire is suppressed to a point at which it will not re-ignite in the
absence of the agent. The time between region 38 and region 40 identifies
the interval during which the fire must be suppressed to prevent
re-ignition.
FIG. 3 graphically illustrates the concentration, in volume percent, of
inerting agent. A minimum concentration of inerting agent, 3.3% by volume
for Halon 1301, is required to suppress the fire up to region 40 and to
prevent re-ignition. Since the inerting agent dissipates with time, a
maximum concentration 42, well in excess of the minimum concentration
required to extinguish the fire at region 38 is provided.
This excess concentration, while necessary to insure suppression, is not
required to inert the fire and may be eliminated by the method of the
invention.
FIG. 4 graphically illustrates the mass flow rate of an inerting agent for
a dual stage fire extinguisher in accordance with the invention. In a
first stage 44, the inerting agent is discharged at a first rate that is
effective to extinguish a fire as indicated by region 38. Subsequent to
extinguishing the fire, the mass flow rate undergoes a transition 46 to a
second mass flow rate 48 that is sufficient to suppress the fire.
As illustrated in FIG. 5, the volume concentration achieves a maximum 42
only slightly above the extinguishing region 38 and then remains
sufficiently high to prevent the fire from re-igniting.
FIGS. 6 and 7 illustrate the savings by the dual stage process of the
invention. In FIGS. 6 and 7, the mass flow rate and volume concentrations
from a single stage fire extinguisher as known from the prior art is
superimposed over the dual stage graphs of FIGS. 4 and 5. The
cross-hatched region 50 represents a savings in the amount of inerting
material required.
FIG. 8 further illustrates the potential inerting agent weight savings
using a dual stage extinguisher system. Dependent on the type of fire and
the burning medium, the percentage of total inerting agent required as an
extinguishing agent can be determined. A lesser quantity of suppressing
agent then constitutes the balance of the inerting agent weight. By
extending a line from the "extinguishing agent required" axis to the agent
weight savings 52 and then extending the line horizontally to the axis
labeled "weight savings," the savings can be calculated.
As an example, if 50% of the weight of the inerting agent in a single stage
extinguisher is required for extinguishing, in the dual stage
extinguisher, only an additional 6% is required for suppression enabling,
as illustrated at reference point 54, a weight savings of 44%.
The dual stage system of the invention is applicable to a pressurized Halon
system as illustrated in FIG. 1. The system reduces the total amount of
Halon required, lessening the environmental impact and extending the
availability of Halon for aircraft fire systems and other applications.
The increased efficiency of the dual stage system of the invention,
facilitates the use of other fire inerting agents, that while less
effective than Halon, are safer for the environment. The inerting agent 28
may be replaced with other agents such as HFC-227 (CF.sub.3 CHFCH.sub.3),
HFC-125 (CF.sub.3 CF.sub.2 H), HFC-236, nitrogen or carbon dioxide. As
illustrated in FIG. 9, a single pressurized cylinder 60 has a flow rate
regulator 62 to provide the proper mass flow rate of inerting gas to the
fire for both extinguishing and suppression.
Alternatively, as shown in FIG. 10, the inerting agent is stored in a first
vessel 64 in a volume and conduit system effective to provide a sufficient
mass flow rate and gas concentration to extinguish the fire. A second
vessel 66 contains either the same inerting agent or a different inerting
agent in an amount and with conduit of a sufficient flow rate to provide
effective inerting agent to suppress the fire. A baffle 68 controls the
flow of the inerting agents to the conduit 30.
Fire suppressing gas generators, as known from U.S. Pat. No. 5,613,562 to
Galbraith et al., that is incorporated by reference in its entirety
herein, may also be utilized. As illustrated in FIG. 11, in the gas
generator system 70, a squib 72 ignites a gas generating chemical mixture
74 that is either, then expelled onto the fire or, directed against a fire
extinguishing powder 76 expelling the powder. Suitable fire extinguishing
powders include magnesium carbonate, potassium bicarbonate, sodium
bicarbonate and ammonium phosphate.
In yet another embodiment, as illustrated in FIG. 12, the gas generator
delivers a gaseous stream to a fire inerting liquid 78 that is preferably
a vaporizable liquid including fluorocarbons, molecules containing only a
carbon-fluorine bond, and hydrogenated fluorocarbons molecules containing
both carbon-hydrogen and carbon-fluorine bonds.
FIG. 13 shows an alternate extinguisher 100 in which the inerting agents
are arranged in series in a single vessel 102. The extinguisher 100 may
fit within a storage location 101 of an aircraft. In a retrofit situation,
the location 101 may be dimensioned for a existing single-agent
extinguisher to be replaced by the extinguisher 100. Specifically, the
vessel 102 is divided by a wall 103 into first (downstream) and second
(upstream) chambers 104 and 106, respectively. The downstream and upstream
chambers contain first and second inerting agents 108 and 110,
respectively, and first and second pressurants 112 and 114, respectively.
The vessel has valved fill ports 116 and 118 in communication with the
first and second chambers, respectively, for filling such chambers with
their associated inerting agent and pressurant.
The wall 103 includes an aperture 120. The aperture 120 is normally closed
such as by a valving element such as a burst disk 122. The aperture is
openable responsive to a pressure difference between the upstream and
downstream chambers 104 and 106. In particular, when the pressure in the
upstream chamber exceeds that in the downstream chamber by at least a
threshold amount, the burst disk opens (either permanently in the case of
a frangible disk or non-permanently in the case of certain spring-loaded
valves or the like). A discharge to conduit 124 extends from a discharge
port in the vessel 102. A valve 126 is positioned between the discharge
conduit 124 and a distribution conduit 128 which directs the inerting
agents to the fire location as discussed below.
In the exemplary embodiment, the vessel 102 is oriented so that the
upstream chamber 106 is above the downstream chamber 104, with the wall
103 extending horizontally. The first inerting agent 108 is selected for
its usefulness in extinguishing the fire. The second inerting agent 110 is
chosen for its usefulness in suppressing the fire and advantageously has
no adverse chemical interaction with the first inerting agent. Exemplary
agents are: HFC's (particularly those approved by the U.S. Environmental
Protection Agency (EPA) under the Significant New Alternative Policy
(SNAP), e.g., HFC-125, HFC-23, HFC-227ea and HFC-236fa); liquids (in
particular water and water-based agents, e.g., water with a freezing point
depressant); CF.sub.3 I; PFC's; and Halon 1301. Preferred pressurants may
be compressed or liquified gases (e.g., compressed nitrogen gas). In the
exemplary embodiment, the pressurants 112 and 114 comprise compressed
nitrogen gas in respective headspaces of the downstream and upstream
chambers above the associated liquid bodies of inerting agents 108 and
110. The pressurants 112 and 114 initially maintain the associated
chambers at gage pressures in an exemplary range of between about 200 psi
(1.4 MPa) and about 1000 psi (6.9 MPa).
In operation, the valve 126 which, for example, may be in the form of a
solenoid-operated valve or a pyrovalve, is caused to open such as by a
command from a human user or from an automated controller connected to a
fire detection system. Once the valve 126 is open, the pressure of the
first pressurant 112 expels the first inerting agent 108 from the vessel
102 through the conduits 124 and 128 to direct the inerting agent to the
fire location. The system dimensions and geometry (in particular the
cross-sectional areas of the conduits 124 and 128 and valve 126) and the
amount and pressure of the pressurant 112 are selected to produce the
desired extinguishing flow rate of the first inerting agent so that the
concentration of the first inerting agent within the fire location quickly
reaches the level required to extinguish the fire during an extinguishing
interval.
As the pressurant 112 expands, expelling the inerting agent 108, the
pressure within the downstream chamber 104 decreases accordingly. Such
pressure eventually decreases to the point where it is below the pressure
in the upstream chamber 106 by a threshold amount. When this occurs, the
aperture 120 is opened such as by a bursting of the burst disk 122. The
second pressurant 114 then drives the second inerting agent through the
aperture 120 and into the downstream chamber 104. This helps drive the
remaining first inerting agent 108 (if any is left) from the downstream
chamber. The flow rate of the second inerting agent from the upstream
chamber to the downstream chamber (and thus via conservation principles
from the downstream chamber to the fire) is limited by the size of the
aperture 120. Advantageously, the aperture 120 has a minimum
cross-sectional area which is substantially smaller than the effective
minimum cross-sectional area of the flow path of inerting agent from the
downstream chamber to the fire. For example, the aperture 120 may have a
minimum cross-sectional area of between about 2% and about 25% of the
effective minimum cross-sectional area downstream of the downstream
chamber. This difference in cross-sectional area, combined with any
difference in the pressurization of the pressurant 114 relative to the
pressurant 112 limits the flow rate of the second inerting agent to a rate
which is advantageously just sufficient to maintain a desired suppression
concentration of the inerting agents in the fire location. The effect is
that the second inerting agent flows through aperture 120 between the
chambers and is expelled from the vessel at a rate and for a time which
are effective to suppress re-ignition of the fire during an interval
whereafter the fire is unlikely to re-ignite in the absence of the
inerting agent(s). When different agents are used as the first and second
agents, the concentration of the first agent will typically decrease
during the suppression stage as it is replaced by the second agent.
When stored under pressure, both inerting agents may be in liquid form as
described above. However, an agent which is stored in liquid form may be
delivered in gaseous form due to the pressure drop between the chamber in
which the agent is stored and the ambient conditions at the point of
delivery. In an exemplary embodiment, the first inerting agent 108 may be
delivered substantially in liquid form while the second inerting agent 110
is delivered substantially in gaseous form. For example, the first
inerting agent could consist essentially of water or a water-based agent
while the second inerting agent consists essentially of the EPA SNAP
approved HFC's identified above or their mixtures.
One key advantage of the system 100 is that it may be configured as a
drop-in or minimal alteration replacement for existing Halon systems. The
vessel 102 may be made substantially to fit within the envelope required
by an existing single chamber Halon vessel. The invention thus allows a
more efficient use of a less efficient agent to replace a less efficient
use of the highly efficient Halon of a prior art system.
While the dual stage fire extinguisher of the invention has been described
in terms of an engine nacelle, it is also effective to extinguish a fire
in other confined areas. Thus, in addition to the illustrated wing
nacelle, the invention may be applied to engine compartments which are
partially or fully integrated into the aircraft fuselage and, beyond
engine compartments, to areas such as cargo dry bays, personnel
compartments of tanks and other armored or non-armored vehicles,
ammunition storage compartments of tanks, ship holds and spacecraft.
Many of the design parameters of the system will be application-dependent.
Influencing factors include: the volume of the region containing the fire;
the expected type of fuel; the expected temperatures of potential
re-ignition sources; the expected rate of dissipation of an accumulation
of inerting agent (which may be influenced by factors such as the speed of
a moving aircraft or other vehicle, and the degree of structural damage
such as increased ventilation due to shrapnel holes, etc.); and the
type(s) of inerting agent utilized.
It is apparent that there has been provided in accordance with this
invention a method for suppressing a fire that fully satisfies the
objects, means and advantages set forth hereinbefore. While the invention
has been described in combination with specific embodiments thereof, it is
evident that many alternatives, modifications and variations will be
apparent to those skilled in the art in light of the foregoing
description. Accordingly, it is intended to embrace all such alternatives,
modifications and variations as fall within the spirit and broad scope of
the appended claims.
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